Conventionally, aluminium is produced by the electrolysis of alumina dissolved in a cryolite based molten salt bath by the more than hundred years old Hall-Heroult process. In this process carbon electrodes are used, where the carbon anode is taking part in the cell reaction resulting in the simultaneous production of CO2. The gross consumption of the anode is up to 550 kg/ton of aluminium produced, causing emissions of greenhouse gases like fluorocarbon compounds in addition to CO2. For both cost and environmental reasons the replacement of carbon anodes with an effectively inert material would be highly advantageous. The electrolysis cell would then produce oxygen and aluminium.
Such an anode will, however, be subject to extreme conditions and will have to fulfil very severe requirements. The anode will simultaneously be subjected to around 1 bar of oxygen pressure at high temperature, the very corrosive molten salt bath specifically designed to be a solvent for oxides, and a high aluminium oxide activity. The corrosion rate must be low enough so that a reasonable time between anode changes is achievable, as well as the corrosion products must not adversely affect the quality requirements of the produced aluminium. The first criterion would mean a corrosion rate not higher than a few millimeters per year, while the second is very dependent on the elements involved, from as high as 2000 ppm for Fe to only a few tens ppm or lower for elements like Sn to fulfil today's requirements for top quality commercial aluminium.
Many attempts have been made to develop inert anodes. The work can be divided into three main approaches; a ceramic material doped to sufficient electronic conductivity, a two or more phase ceramic/metal composite or a metal alloy anode.
Many of the compounds in the first group that much work later have been focused on, were first studied in this context by Belyaev and Studentsov (Legkie Metal.6, No.3, 17–24(1937)) a.o. Fe3O4, SnO2, Co3O4 and NiO and Belyaev (Legkie Metal.7, No.1,7–20(1938)) a.o. ZnFe2O4, NiFe2O4.
Later examples from the first group are anodes based on SnO2 doped with e.g. Fe2O3, Sb2O3 or MnO2 documented in U.S. Pat. No. 4,233,148 (electrodes with up to 79 wt % SnO2) and U.S. Pat. No. 3,718,550 (electrodes with more than 80 wt % SnO2). Sn impurities in the produced aluminium do, however, strongly impair the properties of the metal even at very low concentrations and so render an anode based on SnO2 impractical.
Further, in EP0030834A3 doped spinels are described, with a chemical composition based on the formula MIxMII3−xO4.yMIIIb+On/2 where MI is a divalent metal a.o. Ni, Mg, Cu and Zn, while MII is one or more divalent/trivalent metals from the group Ni, Co, Mn and Fe, and MIII is one or more from a large group of 4- , 3-, 2- and monovalent metals.
Other examples are the range of spinel and perovskite materials described in U.S. Pat. Nos. 4,039,401 and 4,173,518 of which, however, none have proven practical for use in an aluminium electrolysis cell. This is partly because of limited corrosion resistance and partly because of low electronic conductivity.
In U.S. Pat. Nos. 4,374,050 and 4,478,693 is disclosed a generic formula describing compositions of possible anode materials. The formula would cover practically all combinations of oxides, carbides, nitrides, sulfides and fluorides of virtually all elements of the periodic table. The examples concentrate on various stoichiometric and nonstoichiometric oxides of the spinel structure. None of these have proven practical, presumably because of limited stability towards dissolution and low electronic conductivity. In U.S. Pat. No. 4,399,008 a material is described consisting of two oxide phases of which one is a compound of two oxides and the other a pure phase of one of the component oxides.
As the low electronic conductivity of the anode materials has been a problem, a number of efforts have been documented where the aim has been to combine an inert material with an interwoven matrix of a metallic phase. This is the second group mentioned above. General examples are U.S. Pat. Nos. 4,374,761 and 4,397,729. In U.S. Pat. No. 4,374,761 the compositions of the aforementioned U.S. Pat. No. 4,374,050 are described as the ceramic part of a cermet with a metallic phase that can consist of a range of elements. An example from the extensive work carried out on the cermet anodes based on the spinel NiFe2O4 with a Cu or Ni based metal phase is U.S. Pat. No. 4,871,437 describing a production method for making electrodes with a dispersed metal phase. In U.S. Pat. No. 5,865,980 the metal phase is an alloy of copper and silver. The apparent problems with these materials are partly corrosion of the ceramic phase, and partly oxidation and subsequent dissolution of the metal phase under process conditions.
The third group is exemplified by a number of patents on alloys and alloy configurations. The advantage is the high electronic conductivity and the attractive mechanical properties, but common to all metals and metal alloys is, however, that none except the noble metals will be stable towards oxidation under working anode conditions. Different avenues to solve this problem have been followed. U.S. Pat. No. 5,069,771 discloses a method comprising the in-situ formation of a protecting layer made from a cerium oxyfluoride that is generated and maintained by the oxidation of cerium fluoride dissolved in the electrolyte. This technology was first described in U.S. Pat. No. 4,614,569, also for use with ceramic and cermet anodes, but in spite of extensive development work it has so far not found commercial application. One problem is that the produced metal will contain cerium impurities, and thus requires an extra purification process step.
In U.S. Pat. No. 4,620,905 a metal anode that will form a protective layer by in situ oxidation is described. Similarly, U.S. Pat. No. 5,284,562 describes alloy compositions based on copper, nickel and iron where the oxide formed creates a layer that is protective towards further oxidation. International applications WO 00/06800, WO 00/06802, WO 00/06804, WO 00/6805 describe variations of very similar approaches. In U.S. Pat. No. 6,083,362 an anode is described where the protective layer is formed by the oxidation of aluminium on the surface of the anode, the layer being thin enough to still have acceptable electrical conductivity, and being replenished by the diffusion of aluminium through the metal anode from a reservoir in the anode.
Common to all these suggestions is, however, that none offers fully satisfactory solutions to the problem that metals or metal alloys except the noble metals will oxidize under working anode conditions. The formed oxide will gradually dissolve in the electrolyte, the rate of dissolution depending on the oxide formed. In some cases this leads to build-up of oxide layers resulting in low electrical conductivity and high cell voltage, and in other cases gives spalling and excessive corrosion of the anode. In the ideal case the oxide is formed at the same rate as it is dissolved, the rate not being too high for a reasonable lifetime of the anode and causing unacceptable concentrations of impurities in the produced metal. No such system has been demonstrated.